A conventional exhaust gas recirculating apparatus comprises a circulating path for returning or circulating exhaust gas, after combustion, in a discharge path to an intake path, and an EGR (exhaust gas return) value located in line with the circulating path. The EGR valve is controllably opened and closed depending upon an operating condition of engine in order to purify the exhaust gas.
As one example of such apparatus, Japanese Patent Early Laid-Open Publication No. Hei 2-75748 discloses a self-diagnostic apparatus for an exhaust gas recirculating apparatus. The self-diagnostic apparatus comprises a fuel supply stop determining means for determining whether or not the supply of fuel to an engine is currently stopped, an exhaust gas recirculation forced start means for forcedly opening an exhaust gas recirculation control valve when it is determined that the supply of fuel is currently stopped, an intake tube pressure detection means for detecting intake pipe pressure, and a determining means for introducing the intake pipe pressure detected by the intake pipe pressure detection means separately at two time points, i.e., when the exhaust gas recirculation control valve is forcedly opened by the exhaust gas recirculation forced start means and when not opened, and for determining that the exhaust gas recirculating apparatus is in failure, when a difference between the intake pipe pressure introduced at the two different time points is equal to or less than a predetermined value. In this conventional apparatus, the failure diagnosis is positively made.
Japanese Patent Early Laid-Open Publication No. Hei 4-140464 discloses another example. The self-diagnostic apparatus for exhaust gas recirculating apparatus disclosed in this publication comprises an exhaust return control valve located in an exhaust return path and opened and closed by negative pressure, and a negative pressure control means for controlling the negative pressure which is supplied to the exhaust return control valve. The apparatus also includes an intake pressure sensor for detecting a downstream pressure of an intake system relative to a throttle valve, means for forcedly fully closing and opening the exhaust return control valve, means for detecting a variation of the intake pressure caused by the fully closing or opening action of the exhaust return control valve in accordance with the intake pressure sensor, and means for determining whether or not the exhaust return apparatus is in failure in accordance with the variation of intake pressure. In a modified form, the self-diagnostic apparatus for exhaust gas recirculating apparatus includes an exhaust return control valve located in an exhaust return path which is opened and closed by negative pressure, and a diaphragm-type negative pressure control valve for diluting the negative pressure coming from a throttle valve which is nearly in a fully closed position by the atmosphere in order to make it generally constant with the exhaust pressure upstream relative to the exhaust return control valve and then supplying the same to the exhaust return control valve. The apparatus further includes a first pressure detection means for detecting a negative pressure which is supplied to the exhaust return control valve, a second pressure detection means for the exhaust pressure, and means for determining whether or not the exhaust return apparatus is in failure in accordance with the detection value detected by the first pressure detection means or the second pressure detection means. A determination whether or not the exhaust is returned at a proper return rate in order to prevent NOx from being discharged is also performed.
As shown in FIG. 4, a conventional self-diagnostic apparatus for an exhaust gas recirculating apparatus comprises a throttle valve 108 located in line with an intake path 104 of an engine 102, a circulating path 110 for communicating a downstream side of intake path 104 (relative to the throttle valve 108) to an exhaust path 106, whereby an exhaust gas circulates from the exhaust path 106 to the intake path 104 by way of the circulating path 110. An EGR valve 112 is located in line with the circulating path 110. The EGR valve 112 communicates with an exhaust-type modulator 114. This modulator 114 includes a negative pressure path 116 which communicates with the intake path 104 near the throttle valve 108. A first three-way switching valve (VSV) 118 is located in line with the negative pressure path 116. An intake pipe pressure sensor 124 communicates, through a pressure path 122, with the intake path 104 near an intake side opening 120 of the circulating path 110. Sensor 124 and switching valve 118 are both coupled to an electronic control unit (ECU) 130.
To determine whether or not there is a failure in the exhaust gas recirculating apparatus, such as a clogging of the circulating path, the circulation of the exhaust gas is turned off by deactivating the EGR valve 112 for a predetermined time (for example, a few seconds) during a generally constant vehicle speed. The intake pipe absolute pressure is detected at the time when the exhaust gas is being introduced and also at the time when the exhaust gas is not being introduced, so that the determination can be made in accordance with a difference in pressure.
With this prior system, as shown in FIG. 5, an intake pipe absolute pressure Pb1 is detected at the time when the exhaust gas is introduced (i.e., when the EGR valve 112 is turned on). An intake pipe absolute pressure Pb2 is detected at the time when the EGR valve 112 is turned off. An intake pipe absolute pressure Pb3 is detected at the time when the EGR valve 112 is turned on again. The pressure difference is calculated in accordance with the expression .DELTA.Pb=.vertline.Pb3-Pb2.vertline.. The exhaust gas recirculating apparatus is determined to be in an abnormal condition (i.e., in failure) when this pressure difference .DELTA.Pb is less than a predetermined value.
However, the flow of exhaust gas within the circulating path is only a little more than ten percent of the exhaust gas flowing within the exhaust path. Therefore, the pressure difference between the intake pipe pressure at the time when the exhaust gas is being introduced and the intake pipe pressure at the time when the exhaust gas is not being introduced is minimal. As a result, the diagnostic accuracy and reliability are low and thus disadvantageous in view of practical use. Additionally, when an output condition of the engine is taken into consideration, an introduction time and non-introduction time are obtained during a positive torque area (generally constant vehicle speed) and thereafter the intake pipe absolute pressure is detected. Therefore, the function of the exhaust gas recirculating apparatus which purifies the exhaust gas, and the performance of the vehicle are adversely affected during execution of the conventional self-diagnostic apparatus. Improvement is desired.
According to the present invention, there is provided, in order to obviate the above-mentioned inconveniences, an exhaust gas recirculating apparatus having a throttle valve located in an intake path for an engine, a circulating path for communicating a downstream side of said intake path relative to the throttle valve to a discharge path, an EGR valve located in the circulating path, an exhaust pressure-type modulator communicating with the EGR valve, a first negative pressure path communicating the modulator to the intake path near the throttle valve, a first three-way switching valve located in the first negative pressure path, and an intake pipe pressure sensor communicating with the intake path through a pressure path. A self-diagnostic apparatus for the exhaust gas recirculating apparatus is also provided. The self-diagnostic apparatus includes a second negative pressure path for directly interconnecting the intake path and the EGR valve so as to bypass the modulator, a second three-way switching valve is located in the second negative pressure path, and a control unit is provided for controlling the second three-way switching valve which fully opens and closes the EGR valve when the vehicle is decelerating. Variations of the intake pipe pressure caused by the opening and closing action of the EGR valve are detected by the intake pipe pressure sensor in order to determine a deteriorating condition of the exhaust gas recirculating apparatus in accordance with the variations of intake pipe pressure.
The control unit activates and deactivates the second three-way switching valve to fully open and close the EGR valve when decelerating. The variation of the intake pipe pressure caused by the opening and closing action of the EGR valve is detected by the intake pipe pressure sensor in order to determine a deteriorating condition of the exhaust gas recirculating apparatus in accordance with the variation of the intake pipe pressure. Accordingly, the accuracy and reliability are improved by utilizing the large variations of pressure, rather than the minimal pressure differences of the prior art.
The present invention will now be described hereinafter with reference to the drawings.